Membrane separation processes which use a plurality of semi-permeable membranes in series for the separation of one or more, more permeable components from one or more, less permeable components in a feed stream are known in the art. A state of the art example is the "series type two unit separation cell" process as shown in FIG. 1 which recovers both the permeable and non-permeable components at desired purity while using an optimum combination of power and membrane area vis-a-vis any comparable process. Referring now to FIG. 1, a feed stream F2 comprising one or more, more permeable components and one or more, less permeable components is introduced into the high pressure side of membrane separation module M1. M1's high pressure side is separated from its low pressure side by a semipermeable membrane which is selective for the permeation of the more permeable component(s). A first non-permeate stream NP1 which is enriched in the less permeable component(s) of the feed stream is withdrawn from the high pressure side of M1 and introduced into the high pressure side of membrane separation module M2. Similar to M1, M2's high pressure side is separated from its low pressure side by a semi-permeable membrane which is selective for the permeation of the more permeable component(s). A second non-permeate stream NP2 which is further enriched in the less permeable component(s) is withdrawn from the high pressure side of M2 and recovered as a first product stream. A first permeate stream P1 which is enriched in the more permeable component(s) is withdrawn from the low pressure side of M2, combined with the incoming feed stream F1, compressed in compressor C1 and finally recycled to the high pressure side of M1. A second permeate stream P2 which is further enriched in the more permeable component(s) is withdrawn from the low pressure side of M1 as a second product stream.
It is an object of the present invention to recover either or both of the permeable and non-permeable components at desired purity while using an optimum combination of power and membrane area vis-a-vis any comparable process, including the state of the art process shown in FIG. 1.